Field of the Invention
The present invention is related to optical tracking of an object by a video camera or a video/optical sensor, and more particularly, to a system for determining a global orientation of the video camera, when the camera is not fixed.
Description of the Related Art
Various systems for tracking orientation of a video camera are known. For example, a conventional method of tracking an object uses a rigid body position and orientation tracking of a fixed camera and a plurality of markers (sensors) attached to an object being tracked by the camera. Another conventional method uses orientation tracking by means of recognition of the optical image(s) of the environment of the object being tracked. Yet another conventional approach uses augmenting of the optical orientation in the environment with inertial sensors.
The conventional methods also include:                Optical tracking of an object using additional infrastructure: one or multiple cameras with known fixed positions and orientations in the global reference frame;        Inertial measurement of orientation using accelerometers and/or gyroscopes;        Magnetic compassing to supplement inertial measurement;        Inertial measurement augmented with a gyrocompass instead of a magnetic compass;        Optical recognition of the environment, when the environment is mapped in the global reference frame;        Optical recognition of celestial bodies, which are mapped to the global reference frame using the knowledge of global times, almanacs and ephemerides.        
Each of the above methods has its own flaws or limitations. For example, the infrastructure-based optical tracking self-evidently requires the infrastructure. Often, the orientation needs to be estimated without the actual infrastructure available. The inertial sensors (gyroscopes) tend to drift. While the drift is compensated for tilt angles (pitch and roll), yaw angle remains uncompensated, unless supplemented with other means (e.g., magnetic compass, gyrocompass, or optical reference). The magnetic compasses suffer from disturbances, not only from the environment, but from the object itself, if there are ferrous materials, or if electric currents are present, especially variable currents.
Gyrocompasses can only be used on static objects, as they try to measure the Earth's rotation, which is very slow compared to any movement of the object. Optical recognition of the environment features requires for those features to be mapped (recorded) beforehand. Optical recognition of the celestial bodies requires a clear view of the sky, and knowledge of global time and coordinates that are normally obtained using GPS, since the sky is supposed to be visible anyway. Additionally, the optical tracking suffers from marker occlusion, when the constellation cannot be recognized because not all markers are visible to a camera.
Therefore, it is desirable to have an efficient system for tracking objects when the camera is not fixed and the object does not have positional markers attached to it. Accordingly, there is a need in the art for a reliable system and method for tracking objects by using a video camera or other optical sensors.